Elsevier

Survey of Ophthalmology

Volume 47, Issue 1, January–February 2002, Pages 17-26
Survey of Ophthalmology

Major review
The Eye as Metronome of the Body

https://doi.org/10.1016/S0039-6257(01)00282-XGet rights and content

Abstract

Vision is much more than just resolving small objects. In fact, the eye sends visual information to the brain that is not consciously perceived. One such pathway entails visual information to the hypothalamus. The retinohypothalamic tract (RHT) mediates light entrainment of circadian rhythms. Retinofugal fibers project to several nuclei of the hypothalamus. These and further projections to the pineal via the sympathetic system provide the anatomical substrate for the neuro-endocrine control of diurnal and longer rhythms. Without the influence of light and dark, many rhythms desynchronize and exhibit free-running periods of approximately 24.2–24.9 hours in humans. This review will demonstrate the mechanism by which the RHT synchronizes circadian rhythms and the importance of preserving light perception in those persons with impending visual loss.

Section snippets

Circadian Rhythms

Every day at approximately 6:00 a.m. our serum cortisol level peaks.35 How does our body control the spurt of cortisol release so that it occurs at the same time and amount every day? It is thought that our body has an internal clock with a free-running period of about 25 hours51, 98 and that this endogenous rhythm is entrained to the 24-hour (solar) period.24 In addition to cortisol,24, 35, 37, 51, 108 other hormones such as growth hormone,36 melatonin,33, 45 follicle-stimulating hormone, and

Retinohypothalamic Tract

The retinohypothalamic tract (RHT) is a direct afferent pathway from the retina to several hypothalamic nuclei. This tract sends visual signals, including those from the light/dark cycle, to the hypothalamus, which in turn uses this information to pace its clock and synchronize the endogenous circadian rhythm. The RHT has been described in humans80, 81, 82, 84 as well as several experimental animal models, such as the rat,69, 90, 94 guinea pig,69 golden hamster,10, 70, 75, 95 and other rodents.

Hypothalamus

When the SCN is destroyed in rats, circadian rhythms of hormonal release55 and the sleep-wake cycle disappear.28 Transplantation of fetal SCN into the same host restores rhythmical behavior according to the donor's activity.39, 83 The SCN is proposed to be one and perhaps the only pacemaker of the multioscillator system.59 After ablating the SCN in rats, Stephan and Zucker93 demonstrated a loss of circadian rhythm in the rats' drinking behavior and locomotor activity. In a similar study, Moore

Circadian Rhythm Abnormalities in Blind Persons

Environmental cues, such as social interactions, clocks, regular activity, and feeding, might provide the cues necessary to establish normal circadian rhythms in blind individuals. However, fixed-interval feeding in blind rats did not entrain the circadian pacemaker.20 Likewise, many blind persons without light perception exhibit free-running temperature,18 cortisol, and melatonin levels,45, 47 despite having a daily routine.35, 51 Circadian rhythm abnormalities occur and there is a difference

Conclusion

The importance of light entrainment of circadian rhythms is becoming more and more apparent as further information is unraveled regarding the role of SCN in other neuroendocrine functions, such as thyroid hormone, growth hormone, and GnRH. Controversies regarding the critical nature of light/dark cycle input to neuroendocrine systems such as fertility remain unresolved. Future studies comparing large clinical populations with different degrees of vision will be important.

Circadian rhythms are

Method of Literature Search

A Medline/Ovid search was conducted for all database years (1968–present). Search words were: circadian and vision, diurnal and vision, retino-hypothalamic, circadian rhythms, retinohypothalamic tract, diurnal rhythms, visual entrainment, hypothalamus. These searches were limited to English journals and abstracts. From this bibliography we proceeded with a classical search of primary sources. On two occasions we obtained help in translating from the German.

Acknowledgements

The authors have no commercial or proprietary interest in any product or concept discussed in this article. They wish to acknowledge Mona Khan, MD, for having persistently questioned and researched many of the issues of light entrainment.

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